Resolution:
The rule of thumb is to rebuild deployment platform on the target machine(s) (where formflows have to be deployed), if there are any changes to the bwworkflow/5.1/bui folder or any hotfix is applied.
Deployment platform creation is not related to the Workflow/InConcert server as such (be it primary or secondary).
You should recreate deployment platform on any machine that has the Workflow Desktop component of BWWF installed and where formflow will be deployed. Also before you recreate the deployment platform war file, please make sure that you have the updated files (be it hotfix or updated templates under bwworkflow/5.1/bui) on the machine.